The invention relates to the field of molding fluid flow components. More particularly, it relates to molding a fluoropolymer sweep elbow with straight threadable ends while minimizing distortion of the fitting.
In semiconductor manufacturing facilities, commonly known as fabs, highly corrosive ultrapure fluids are utilized in the manufacturing operations. Conventional plastics and metallic tubing and plumbing components are highly unsuitable in these applications. The materials of choice for plumbing components are fluoropolymer plastics such as PFA and PVDF because of their chemical inertness and nonwettability. The plumbing components will typically comprise tubing, valves, fittings, filters, sensors, tanks and the like.
It is absolutely critical that the connections between plumbing components have an extremely high level of integrity and strength. Typically, the pressure rating of plumbing used in such applications is 75 psi. Loss of joint integrity while carrying highly corrosive fluids at such pressures can result in serious bodily injury, destruction of property and environmental hazards.
The fluids that are handled may comprise slurries with suspended abrasive materials utilized for polishing semiconductor wafers. Such polishing must be carefully controlled and uniform over the surface of the wafer. It is desirable for plumbing that handles such slurries to have transitions that are as smooth as possible to minimize the formation of eddies or quiescent zones in which the suspended material may fall out of suspension. This avoids creating the possibility of a non-uniform distribution of abrasive and consequent uneven polishing of wafers. Uneven polishing creates unusable wafers and loss of yield. Thus, sweep elbows, which have a large turning radius relative to tubing diameter, are preferred to change the direction of fluid flow smoothly to keep suspensions uniform.
From a performance standpoint sweep elbows are preferable to conventional elbows, which may have a turning radius approximately equal to the tubing diameter. However, due to difficulties in molding fluoropolymer fittings such a sweep elbow has not been available. The fluid passageway in molded fittings is typically formed by a core which retracts out of the mold after injecting plastic into the cavity. Applicant is aware of no one that makes a fluoropolymer molded sweep elbow with flared connector ends. This is primarily due to the difficulty of maintaining the straightness and roundness of the nose portion of the fitting when the curved mold core is retracted from the molded part. Such distortion is unacceptable from a quality assurance standpoint since it can seriously reduce the integrity of any connection made with the nose. It is preferable that elbow ends are straight, that the elbow is not overly long and that it can be threaded on both ends. Current practice uses a one piece mold core or insert that distorts the elbow end as the insert is withdrawn from the part making straight ends impossible.
The conventional elongate nose of PFA flared fittings has been useful in that the extended surface area provides more circumferential sealing area and provides adequate structural support to resist bending moments at the fitting. These characteristics contribute to the integrity of the fitting seal under varying and hostile conditions.
An object of the invention then, is to manufacture a fluoropolymer sweep elbow that has straight elongate ends sufficient to provide adequate structural support and sealing surface area. An additional object is to minimize distortion of fittings that lowers quality and reduces yield of product. Another object of the invention is to provide sufficient straight length at the ends of the fitting to support threading to achieve a good threaded seal.
The present invention discloses a method and apparatus for producing molded fluoropolymer sweep elbows and other sweep fittings with straight threadable ends while minimizing distortion of the fitting. The apparatus comprises a mold core or insert that is comprised of multiple curved segments which are retracted from the mold individually. The mold core is divided into two or more pieces on a curved plane parallel to the curved central axis of the core and the segments thus formed are retracted from the mold by a combination of arcuate and translational motions to avoid distortion of the fitting ends.
The fluid passageway in molded fittings is typically formed by a core which retracts out of the mold after injecting liquid polymer into the cavity and allowing the plastic to at least partially set. In the present invention the core generally comprises an arcuate cylinder or segment of a toroid which has a greater and a lesser curvature. The core occupies the center of the mold to create the fluid passageway of the fitting. The core is divided into two or more pieces on a line parallel to the greater and lesser curvatures and between them. In the case of a two piece core each piece will generally have the shape of an arcuate hemicylinder. In this example a two piece core will be discussed. In some applications it may be desirable to divide the core into more segments. After the mold is filled, by injection, with liquid polymer and the polymer has at least partially set, the segment comprising the greater curvature of the core is retracted from the mold in an arcuate motion. After being completely retracted the greater segment is translated away from its center of rotation to provide clearance for the motion of the lesser segment. Subsequently, the lesser segment of the core is translated a small distance away from its center of rotation to free it from contact with the lesser curvature of the fluid passageway of the molded fitting. The lesser segment of the mold core is then retracted arcuately from the fluid passage of the fitting without the lesser segment further touching the molded item. Lastly, the female mold is opened to free the finished fitting.
A 90-degree sweep elbow is used as an example here. This should not be considered as limiting. It should be understood that the process and apparatus as described herein may be utilized to manufacture different types of fittings such as sweep elbows, sweep Ys, sweep Ts, sweep Us, traps and other fittings associated with valves, gauges, flow meters and other equipment and apparatus used in fab plumbing installations utilizing fluoropolymer materials.